USN-1463-2 fixed a bug in Unity 2D exposed by a recent Firefox update. It
was discovered that the issue was only partially fixed on Ubuntu 11.04.
When Thunderbird was started from the launcher, Thunderbird was still
unable to obtain pointer grabs under certain conditions. This update fixes
the problem.

Original advisory details:

USN-1463-1 fixed vulnerabilities in Firefox. The Firefox update exposed a
bug in Unity 2D which resulted in Firefox being unable to obtain pointer
grabs in order to open popup menus. This update fixes the problem.

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 11.04:
unity-2d-launcher 3.8.4.1-0ubuntu1.2

After a standard system update you need to restart your Unity 2D session to
make all the necessary changes.